Nails vs. Screws: When and Where to Use Them in Post-Frame Construction
December 8, 2015
To guarantee quality in your post-frame construction, the devil is in the details. The use of nails vs. screws in your building is a perfect example. Knowing when, where and why each fastener should be used is yet another step towards assuring you get the most bang for your buck.
The use of nails or screws typically comes down to one over-riding issue: Money. Screws, without a doubt, provide a more secure connection. Nails, however, are less expensive and faster when using nailguns.
Both screws and nails have their place. In this post, we’ll touch on the best application for each respective fastener.
Wood-to-Wood: Using Nails “Under” the Steel
Whether you’re applying nails or screws, the goal is to provide enough shear strength to support wind and snow loads. Nails, when applied in the correct manner and number, can provide the necessary strength. They are best used in three specific areas “under” the steel that wraps the structure (we’ll get to that in the section on screws.)
* Purlins to Trusses: Much like the girts form the framework for the steel on the walls, the purlins do the same on the trusses where the roof steel is attached.
* Bracing: During construction bracing keeps building frame plumb and straight prior to structural sheathing. Permanent bracing stiffens the structure and helps the structure withstand wind and snow loads. Bracing is used to connect columns, girts, purlins and trusses. Properly sized and placed nails are used to connect these lumber bracing elements.
A couple added tips about using nails:
Ring-shank nails: A ring-shank nail will provide even more strength than typical nails. The circular pattern embedded in the nail locks into the wood fiber when it’s driven into place. Wick uses only ring-shank nails.
Galvanized nails: Much like builders can cut corners on the use of nails versus screws, another way to shave money is to skimp on the quality of the nails. In areas subject to corrosion like buried columns, splash planks and anywhere a nail is exposed to weather, galvanized nails should be used.
Using a less expensive nail shouldn’t be a financial decision. It should be based first and foremost on the requirements dictated by the engineers designing the building, who can specify the type of strength you need.
Steel-to-Wood: Securing Your Steel Structure
Always use screws to attach your exterior steel siding to a structure. The standard screws that we use provide over twice as much pull-out resistance as nails – 122% to be exact.
Why is this so critical on the steel that encases your building?
First, the obvious reason is that steel that wraps your post-frame structure is your outermost material. It’s the front-line against the elements. Over time, moisture, temperature, wind and snow loads will cause your building to move.
Second, the steel not only provides a barrier to the elements, it also ties the entire structure together and gives it strength. If that entire “shell” is fastened with maximum resistance, your building will be stronger and have a longer lifespan. As mentioned above, Mother Nature puts forces on the nails and screws causing movement. The more those nails back out, the more your structure will move.
Let’s get into a few guidelines on what types of screws to use, and where.
Greater Resistance for Longer Screws: The longer the screw, the more likely it is to control contraction and expansion of steel. This is especially critical when attaching roof steel due to the length of the panels. Wick uses 1-½” screws to attach the steel panels to the roof. These screws provide an uplift load resistance 56.5% greater versus a 1” screw. Again, we are always cautious of diminishing returns, but these numbers make our decisions easy.
Screwing in the Flat Versus the Rib: Some builders will install screws on top and through the major rib of your steel siding, versus in the flat. They’ll use a 2” screw and install on the top of the rib. Unfortunately, the uplift resistance is lower, because only an inch of the screw goes into the wood (see visual below.) This also makes the panel more susceptible to moving when wind and snow exert stresses on your building, and when the steel panel expands and contracts with heat and cold.
By screwing in the pan (the lower part of the steel), you gain 56% greater uplift resistance, as 1 ½” of the screw sinks into the wood. You also get less movement due to wind, snow and temperature stresses.
The argument against using the screw in the flat or pan especially on the roofline, is that water runs through those channels, and has greater potential for leaking at the screw. That’s a problem easily overcome by using the appropriate screw with an integral washer head design which reduces overdriving and ensures a weather tight seal.
Use Data to Make Your Decision
We’ve mentioned the point of diminishing returns numerous times throughout this post. The right engineer can use data to determine where that cut-off is, and ultimately can make the nail vs. screw decision an easy one.